---
title: "Static channel assignment and CSMA/CD"
subject: "Computer Networks and Security"
chapter: 3
key: "semester_7/computer_networks_and_security:static-channel-csmacd"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/static-channel-csmacd
site: BCT Engineering
---

# Static channel assignment and CSMA/CD

Computer Networks and Security · Chapter 3: Data Link Layer

- Asked in: 2073 Chaitra, 2+6 marks
- Video: [Lec-35: Carrier Sense Multiple Access in Computer Network || CSMA || Computer Networks](https://www.youtube.com/watch?v=IftFvfSywCQ) — Gate Smashers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/static-channel-csmacd
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-3

## Question

Why do you think that static channel assignment is not efficient? Explain about the operation of Carrier Sense Multiple Access with Collision Detection.

## Short answer

Static allocation (FDM/TDM) wastes idle shares and increases delay N-fold for bursty traffic. CSMA/CD works by listening before sending; if idle, transmit; detect collision by comparing transmitted and received signals; on collision, send jam and backoff using binary exponential backoff.

## Answer

#### Why static channel assignment is not efficient
In static assignment (FDM/TDM) the channel is split into N fixed parts, one per user.
- When a user is idle, its share is wasted and no other user can use it.
- Computer traffic is **bursty** (long idle periods with short peaks), so most reserved capacity is unused.
- Delay increases N times (T_FDM = N·T) compared with one user having the whole channel.
- It does not adapt when the number of users changes.
Dynamic allocation protocols such as CSMA/CD avoid this waste.

#### CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
Used in classic Ethernet (IEEE 802.3).

**Operation**
1. **Carrier sense:** before sending, a station listens to the channel.
2. If idle, it transmits (1-persistent: it sends immediately when idle). If busy, it waits until the channel becomes idle.
3. **Collision detection:** while transmitting, the station keeps listening. If the signal on the line differs from what it sends, a collision has occurred.
4. On collision, the station stops and sends a short **jam signal** so everybody knows.
5. **Binary exponential backoff:** after the nth collision, it waits a random number of slot times chosen from 0 to 2ⁿ − 1 (limit 2¹⁰) and tries again. After 16 attempts it gives up.

```text
 Ready --> Sense --busy--> wait
             |idle
          Transmit --collision--> Jam --> Backoff --> Sense
             |no collision
           Done
```

**Minimum frame size:** the frame must be long enough that the sender is still transmitting when a collision signal returns, i.e. transmission time ≥ 2 × propagation delay. Ethernet uses a 64-byte minimum frame.

CSMA/CD works well for low and medium load, but has performance loss at heavy load due to many collisions.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is static channel assignment inefficient for computer traffic?
   A. Traffic is steady, so fixed shares overload
   B. Traffic is bursty, so idle users' reserved shares are wasted
   C. It causes more collisions
   D. It needs token passing
   Answer: B. Bursty traffic leaves most reserved capacity unused and idle shares cannot be used by others. (Key point: Why static fails)

2. How does a CSMA/CD station detect a collision?
   A. It receives a NAK
   B. It keeps listening and sees the line signal differs from what it sends
   C. A timer expires
   D. The receiver sends a jam signal first
   Answer: B. While transmitting, the station compares the line signal with its own; a difference means a collision. (Key point: CSMA/CD collision detection)

3. After the nth collision, how long does a CSMA/CD station wait?
   A. A fixed n slot times
   B. A random number of slots from 0 to 2^n - 1
   C. Exactly 2^n slots
   D. Until a token arrives
   Answer: B. Binary exponential backoff picks a random wait between 0 and 2^n - 1 slots (limit 2^10), giving up after 16 attempts. (Key point: Backoff)

4. Why must an Ethernet frame have a minimum length?
   A. To make the CRC work
   B. So the sender is still transmitting when the collision signal returns (transmission time >= 2 x propagation delay)
   C. To fit the jam signal in the header
   D. To limit buffer use
   Answer: B. Otherwise the sender could finish before learning of a collision and would not detect it. (Key point: Minimum frame size)

## Related in Computer Networks and Security

- [Short note: CSMA/CD](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Acsma-cd-note) — Computer Networks and Security, chapter 3
- [Multiple access protocols](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Amultiple-access-protocols) — Computer Networks and Security, chapter 3, asked 2×
- [Multiple access in IEEE 802.5](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aieee-802-5-access) — Computer Networks and Security, chapter 3
- [Media Access Control and significance](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Amac-significance) — Computer Networks and Security, chapter 3

## Related in other subjects

- [Ethernet with CSMA and CSMA/CD](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aethernet-csma-cd) — Telecommunications, chapter 8
- [Frequency reuse and channel assignment strategies](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afrequency-reuse-channel-assignment) — Wireless Communication, chapter 2
- [Multiple access and near-far effect mitigation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-near-far) — Wireless Communication, chapter 7
- [Frequency hopped multiple access (FHMA)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-operation) — Wireless Communication, chapter 7, asked 5×